The present invention relates to a method and apparatus for reducing the liquid content of sugar cane bagasse as it emerges, for example, from a sugar cane diffuser comprising a linear or circular diffuser construction. Such a diffuser may be equipped with a stationary or with a movable screen bottom as is well known in the art, see for example, German Pat. No. 1,567,245.
The just mentioned German Pat. No. 1,567,245 discloses the use of three cylinder high pressure mills arranged downstream of a diffuser as viewed in he feed advance direction of the bagasse. Such high pressure mills reduce the moisture content of bagasse to a remaining moisture content of about 50 to 52% by weight at a substantial expense for the original equipment investment as well as for the power requirements for operating such high pressure mills.
Referring first to FIG. 4 representing the prior art as disclosed in German Patent No. 1,567,245 such a prior art system comprises a diffuser 20 followed by one or several high pressure mills 30. The bagasse B travels through a diffuser output trough 25 having a bottom 21 formed by a screen type conveyor belt 22 feeding the bagasse B toward a peeling drum 28 which simultaneously forms a closure cylinder or roller at the end of the diffuser output trough 25. The peeling or closing drum 28 is located above an unperforated bottom portion 21' of the trough 25. The unperforated bottom portion 21' is located just downstream of a guide roller 24 over which the conveyor belt 22 travels. A juice collecting trough 23 is arranged below the upper run of the conveyor belt 22. A so-called pendulum roller or cylinder 26 is supported by lever arm 27 and provides a preliminary compaction of the bagasse B above the screen bottom 21 for a preliminary dewatering.
The closing and peeling roller 28 feeds the partially dewatered bagasse onto a conveyor 29 which in turn supplies the bagasse into the first high pressure roller mill 30. The output of the first roller mill 30 is supplied to a conveyor 37 which in turn feeds the further dewatered bagasse into the second high pressure roller mill 30. Both high pressure roller mills are of the same construction and therefore the same elements are designated by the same reference numbers and only the first mill will be described. Each mill has a housing 31 supporting two lower cylinder cores or shafts 32 surrounded by high quality steel jackets 33 and separated by a so-called bagasse bar 34. A further upper roller core 35 is also supported in the housing 31 and surrounded by a high quality steel jacket 36.
The pendulum roller 26 forming part of the diffuser 20 causes a preliminary dewatering. The further dewatering of liquid removal is then accomplished sequentially in the high pressure three roller mills 30. The disadvantages of such mills are well known. For example, the roller jackets 33 and 36 have an outer diameter up to 1000 mm and are made of a special casting alloy. The roller cores or axles 32 and 35 must also be made of high strength steel and the shrinking of the jackets onto the cores or axles is also a costly operation. Due to the high pressures involved ranging to approximately 1000 kg/cm.sup.2, the roller jackets and also the cores are subject to high wear and tear so that repairs or a complete exchange is necessary from time to time. Heretofore sugar factories have been equipped with special repair shops for the set up and maintenance of these high pressure mills in order to minimize dead times necessitated by such maintenance and exchange work.
Further, counting the pendulum roller 26, systems of the prior art as disclosed in the above mentioned German Pat. No. 1,567,245, require a total of seven rollers of which the six rollers in the two mills must be made of the mentioned special steel alloys in order to withstand the high pressures in the order of 1000 kg/cm.sup.2.
Another disadvantage of prior art three roller high pressure mills is seen in that their structure inherently requires the so-called bagasse bar 34 which separates the two lower rollers 32, 33, however, which is without any effect for the liquid removal for all practical purposes. At the same time, these bagasse bars substantially increase the required power input for driving these mills because of the large frictional sliding forces of the bagasse along these bars 34. Thus, it is known that the driving powers for each of these mills requires approximately 1000 PS (horsepower). Additionally, the initial investment of capital for these high power drive means with the required reduction gears are quite substantial. For example, where the prime mover is a steam turbine, the gear reduction ratio may be 1 to 2500 and more.
The advances which have been made over the past 50 years in the development of three roller high pressure mills for the squeezing-out of sugar cane bagasse related primarily to the ever increasing roller pressures. Today such pressures have reached values of about 1000 kg/cm.sup.2 between the rollers as mentioned. More recent developments go even further in this wrong direction in that five roller high pressure mills have been built as over-dimensioned blocks again operating with pressures of about 1000 kg/cm.sup.2.
A theoretical investigation regarding the internal pressure and time lapse functions in a three roller high pressure mill of conventional construction has been made taking into account the above mentioned 1000 kg/cm.sup.2 pressures between the rollers. Such theoretical investigation has taken into account the compression characteristics of sugar cane bagasse according to the publication by Noel Deer entitled "Relation Between Pressure and Compression" published by Elsevier Publishing Comp., Amsterdam, 1972 (147). As a result of said theoretical investigation the conclusion has been reached that these high pressures between the rollers are quite without effect in such three roller high pressure mills due to the pressure and lapse of time functions in the order of merely 100ths of seconds in the upper pressure rise range.
The numerically important result of said theoretical investigation is seen in that in conventional high pressure mills having a customary circumferential roller speed of, for example, 25 cm/s the time available for the rise of the roller pressure from 49.4 kg/cm.sup.2 to 727 kg/cm.sup.2 is only 5/100 s. This figure applies to a roller diameter of 800 mm. As a matter of fact, for a rise in the pressure from 83.9 to 727 kg/cm.sup.2 the available time is only 1.6/100 s. Such a short time is insufficient to achieve an efficient squeezing-out of all structural components of the sugar cane bagasse because of the widely varying hardness of the bagasse components and because of the wide variations in the structure of the sugar cane bagasse. It has been found that the high compressions employed today in three roller high pressure mills are not effective for the squeezing-out, but are rather converted into heat during the short available time periods. The inefficiency of such high pressures is further evidenced externally by the well known loud and clearly audible sporadic so-called juice shots which sound like an explosion and which are evidence of the non-uniform, uneconomical, and substantially inefficient squeezing-off operation in prior art high pressure mills.
The theoretical considerations have further shown that the high pressures result in a large increase in the hardening of the sugar cane bagasse whereby the required remaining moisture content cannot be achieved by the substantially instantaneously established maximum pressure which is reached in a matter of a few hundredths of a second.
The preliminary dewatering in a system as shown in FIG. 4 is accomplished by means of the pendulum roller 26 which simultaneously operates as a trough closing roller. The roller 26 has a diameter of 3 to 4 m. Such a roller 26 operating as a dewatering roller and as a trough end closing roller may be operated only with pressures of approximately 1.0 kg/cm.sup.2 because even these low pressures result in very large, hardly controllable frictional forces due to the large roller diameter and due to the respectively large pressure application surface area. Additional friction forces are caused, for example, in a linear diffuser trough by the stationary screen bottom which is equipped with chains which transport the bagasse through the diffuser trough by means of entraining rods. Several such chains are necessary in each diffuser and the loads effective in each of these several chains which transport the bagasse into the diffuser amount to approximately 35,000 kg. Approximately 30% of these forces are caused alone by the friction of the roller 26 even at the mentioned low pressure. The sum of all the chain forces in a diffuser having, for example, a 4000 ton sugar cane capacity per day amounts to 200 tons and more. Such chains cost approximately $225,000.00 and the total price of a conventional diffuser amounts to approximately 1.15 million dollars not counting the high pressure mills, each of which costs approximately also 1.15 million dollars. These figures show the large economic importance in the reduction of the chain forces by avoiding the prior art type dewatering in the diffuser itself above the diffuser screen bottom as described in the above mentioned German Pat. No. 1,567,245. The large chain strength required according to the prior art also cause a very heavy overall construction because the chain guide rollers, the chain drives, and the chain gear system all must be constructed with due regard to these large chain forces. The invention aims at avoiding such large chain strengths. German Patent Publications Nos. 2,657,232 and 2,716,666 as well as 2,819,719 disclose efforts in improving the low pressure preliminary dewatering in a diffuser by means of a roller such as shown at 26 in FIG. 4. Such improvements involve the use of differently shaped screen surfaces which may be placed at different elevational positions in a roof type sequential arrangement. Such additional screen surfaces may also be bent into the desired shape. These arrangements have increased the effectiveness of the preliminary dewatering of the bagasse prior to its final squeeze-out in the following high pressure mills. However, one overriding disadvantage of such additional screen surfaces is seen in that they even increase the frictional forces as compared to the roller 26 disclosed in German Pat. No. 1,567,245. Although these additional screen surfaces result in a desirable relief of the final high pressure squeeze-out in the three roller high pressure mills, the costs for these additional devices are out of proportion to their advantages.